1. Field of the Invention
The present invention relates to a device for measuring the thickness of a form document set in an impact printer, such as a dot line printer, and more particularly to a form thickness measuring device for measuring thickness of a continuous form consisting of a single or plural stacked sheets of paper.
2. Description of the Related Art
Conventional impact serial printers include a print head and a platen for supporting a print paper or a form document thereon. The gap between the print head and the platen is determined depending upon the thickness of the form document so that hammers mounted on the print head can make impressions of dots on the form supported on the platen.
It should be noted that not only a form with a single sheet of paper but also a form with plural stacked sheets of paper can be used in the impact printers.
Unlike impact serial printers, dot line printers print on form documents by striking hammers, which are mounted on a 13.6 inch width print head, against the forms with a relatively strong force. Therefore, the gap between the print head and the platen needs to be precisely determined depending upon the thickness of the form. Nevertheless, there has been no form thickness measuring device for use in the dot line printer, that is low in manufacturing cost, simple in construction, and easy to operate.
In view of the foregoing, it is an object of the present invention to provide a thickness measuring device capable of measuring a thickness of a form having a variable thickness, which is low in cost and high in accuracy.
To achieve the above and other objects, there is provided a thickness measuring device including a substrate; a stepping motor supported on the substrate and having a rotor shaft forwardly and reversely rotatable about its own axis; a translating mechanism for translating rotations of the rotor shaft into a linear movement; a pressing block formed with a projection, the pressing block being coupled to the rotor shaft through the translating mechanism for moving the projection toward and away from an object to be measured supported on a supporting plate in accordance with forward and reverse rotations of the stepping motor; and a control unit for controlling the stepping motor to move the projection from a predetermined fixed position to the object and then to backwardly move the projection from the object to the predetermined fixed position, and for computing a thickness of the object based on an actual amount of rotations of the stepping motor detected when the projection is moved a one-way distance between the predetermined fixed position and the object.
The control unit includes a non-volatile memory storing therein a reference amount of rotations of the stepping motor corresponding to a distance between the predetermined fixed position and the supporting plate, and computing means for computing the thickness of the object based on a subtracted amount of rotations obtained by subtracting the actual amount of rotations from the reference amount of rotations.
The control unit controls the stepping motor to move the projection from the predetermined fixed position to the supporting plate and then to backwardly move the projection from the object to the predetermined fixed position, and the control unit further includes detection means for detecting the reference amount of rotations of the stepping motor by moving the projection a one-way distance between the predetermined fixed position and the supporting plate. The detection means stores the reference amount of rotations in the non-volatile memory.
A first set of data including the actual amount of rotations and the reference amount of rotations is detected while rotating the stepping motor at a first pulse rate. The computing means further computes a first subtracted amount of rotations based on the first set of data. The control unit further includes comparison means for comparing the first subtracted amount of rotations with a first reference value, and determining means for determining that the object falls into a first range of thickness when the comparison means indicates that the first subtracted amount of rotations is less than the first reference value and that the object is out of the first range of thickness when the comparison means indicates that the first subtracted amount of rotations is equal to or greater than the first reference value.
A second set of data including the actual amount of rotations and the reference amount of rotations is detected while rotating said stepping motor at a second pulse rate lower than the first pulse rate. The computing means further computes a second subtracted amount of rotations based on the second set of data. The comparison means further compares the second subtracted amount of rotations with a second reference value greater than the first reference value, and the determining means further determines that the object falls into a second range of thickness when the comparison means indicates that the second subtracted amount of rotations is greater than the first reference value but less than the second reference value and that the object falls into a third range of thickness when said comparison means indicates that the second subtracted value is greater than the second reference value. The values in the first range of thickness is smaller than values in the second range of thickness, and the values in the second range of thickness is smaller than values in the third range of thickness.
When the determining means determines that the object falls into the third range of thickness, the control unit controls the stepping motor to rotate at a third pulse rate lower than the second pulse rate to compute the thickness of the object.
The control unit computes the thickness of the object based on the first subtracted amount of rotations when the determining means determines that the object falls into the first range of thickness. The control unit computes the thickness of the object based on the second subtracted amount of rotations when the determining means determines that the object falls into the second range of thickness.
A third set of data including the actual amount of rotations and the reference amount of rotations is detected while rotating the stepping motor at the third pulse rate lower than the second pulse rate. The computing means further computes a third subtracted amount of rotations based on the third set of data, and the control means computes the thickness of the object based on the third set of data.
In accordance with the thickness measurement of the invention, the thickness of the object can be obtained with high accuracy because the measured thickness is free from influence of resilient deformation of the supporting plate supporting the object to be measured.
According to another aspect of the present invention, there is provided a printer including a print head, a pin feed tractor, and a form thickness measuring device. The pin feed tractor feeds a form document past the print head, and has a tractor plate and a tractor cover arranged in parallel with each other to form a gap therebetween into which the form document is inserted.
The form thickness measuring device includes a substrate; a stepping motor supported on the substrate and having a rotor shaft forwardly and reversely rotatable about its own axis; a translating mechanism for translating rotations of the rotor shaft into a linear movement; a pressing block formed with a projection, the pressing block being coupled to the rotor shaft through the translating mechanism for moving the projection toward and away from the form document supported on the tractor cover through an opening formed on the tractor plate in accordance with forward and reverse rotations of the stepping motor; and a control unit. The control unit controls the stepping motor to move the projection from a predetermined fixed position to the form document and then to backwardly move the projection from the form document to the predetermined fixed position. The control unit further compute a thickness of the form document based on an actual amount of rotations of the stepping motor detected when the projection is moved a one-way distance between the predetermined fixed position and the form document.
The control unit further controls the stepping motor to move the projection from the predetermined fixed position to the tractor cover and then to backwardly move the projection from the tractor cover to the predetermined fixed position. The the control unit includes detection means for detecting a reference amount of rotations of the stepping motor by moving the projection a one-way distance between the predetermined fixed position and the tractor cover, a non-volatile memory, the detection means storing the reference amount of rotations in the non-volatile memory, and computing means for computing a difference between the actual amount of rotations and the reference amount of rotations, and computing the thickness of the form document based on the difference.
The control unit further comprises pulse rate changing means for changing a pulse rate of the rotations of the stepping motor, the pulse rate changing means changes the pulse rate based on the difference.
The pulse rate changing means decreases the pulse rate used for moving the projection from the predetermined fixed position to the form document and to the tractor cover when the difference is greater than a reference value.
According to another aspect of the present invention, there is provided a method of measuring the form thickness.